Slow-speed motion control (&lt;15 RPM rotational and &lt;1.0 IPS linear) is required for many web transport applications such as coating, printing and scanning. In many cases, the application is sensitive to deviations from a nominal operating velocity. These undesirable velocity changes are often referred to by names such as flutter, jitter and wow. The traditional approach to minimizing these fluctuations is to utilize a servomechanism as the motion control system. These servomechanisms utilize closed loop control methods in conjunction with components including controllers, amplifiers, motors, transmissions and feedback sensors. A state-of-the-art servomechanism block diagram is illustrated in FIG. 1. It shows the typical interconnection relationship between the major components.
The basic theory of operation of these servomechanisms is to use a digital controller 10 (usually a digital micro-controller, microprocessor or digital signal processor) to control the motion of a load 26. The digital controller 10 includes a command generator 12 that produces a command signal. The command signal is in the form of a digital count that relates to the rotational or translational position of the load 26. The digital controller 10 uses a phase detector 14 to compare the command signal to a feedback count that comes from a digitizing device 30. This comparison is usually a simple subtraction that produces an error count. This error count is then filtered with a compensation filter 16 (usually some type of lead-lag or proportional-integral-derivative (PID) compensation algorithm) which is required to stabilize and enhance the performance of the overall control loop. The output of the compensation filter 16 is then converted into the analog control voltage in a digital-to-analog converter (DAC) 18 which produces a voltage control signal. This voltage signal drives the motor amplifier 20/motor 22 combination so that the motor 22 can deliver a force or torque to a given load 26. The delivery of this force or load is usually done by means of a transmission 24.
The load 26 then rotates or translates as a result of the applied torque or force. A feedback sensor 28 senses this output rotation or translation of the load 26. Usually, this feedback sensor 28 is an incremental optical encoder. Basically, the incremental optical encoder 28 outputs a pair of square waves voltage signals that are out of phase by 90 degrees. This pair of square waves is normally referred to as quadrature signals due to the fact that they can be measured so as to effectively increase the resolution of the incremental optical encoder by a factor of 4.times.. Whether the servomechanism is measuring only one of the square waves for 1.times. resolution or both square waves for 4.times. resolution, a digitizing device 30 such as an up-down-counter is used to count the rising and falling edges. The result of this counting is the feedback count that is fed back into the controller so that it can be used by the phase detector 14 to produce the error count.
The sequence of operations happens in real time and can be optimized by varying many of the parameter values that exists within the overall system. This control technique is sometimes referred to as quadrature control (if the feedback sensor is an incremental optical encoder) due to the counting technique used in converting the signal from feedback sensor 28 to an appropriate feedback signal for the digital controller 10.
Within this control scheme, each individual component has critical functions and thus must be designed with care. One component that can cause a number of problems if not carefully designed is the transmission 24. There are two primary advantages for using a transmission 24 in a slow-speed application. First, most available servomechanisms are designed for medium (15 RPM to 1000 RPM) or high-speed (&gt;1000 RPM) applications. Therefore, the transmission 24 can provide a means of speed reduction through the effective "gear ratio" of the transmission 24. This speed reduction can be mathematically described by: EQU V.sub.out =V.sub.in /n (1)
where: V.sub.out =velocity of load PA1 V.sub.in =velocity of motor PA1 n=effective "gear ratio" of transmission 18 (usually &gt;1). PA1 where: T.sub.out =torque or force outputted by transmission 24 PA1 T.sub.in =torque produced by motor PA1 n=effective "gear ratio" of transmission 24 (usually &gt;1). PA1 The transmission 24 itself has a finite size that reduces some of the advantage of a smaller motor 22. PA1 Mechanical errors are introduced into the servomechanism by the transmission 24. PA1 The servomechanism of the present invention allows bi-directional motion control to be achieved at all speeds ranging from 0 RPM up to the maximum capabilities of the motor and load combination. PA1 In the slow-speed operation mode, smooth velocity control is achieved by using an angle decoder and cycle counter combination which effectively increases the overall resolution of the base incremental optical encoder by interpolating the sine and cosine signals generated by the encoder. PA1 Optical encoders used with the present invention are widely available and are relatively inexpensive. PA1 The servomechanism of the present invention offers highly accurate position control when operated in the slow-speed operation mode due to the effective high resolution from the angle decoder and cycle counter interpolation. PA1 The resolution of this position control is limited by the number of sine and cosine cycles per revolution of the encoder and an internal controller gain referred to as the angle decoder gain. The encoder and analog-to-digital converter (ADC) specifications limit precision or accuracy of the positioning systems. PA1 The elimination of a transmission simplifies the mechanical design that in turn allows higher performance to be achieved due to less mechanical tolerance errors and easier to control structural dynamics. PA1 The present invention uses a fast angle decoder and cycle counter in order to prevent a motor runaway phenomenon from occurring thus allowing slow-speed control to be performed under conditions in which the servomechanism is experiencing disturbances.
Second, the transmission 24 provides a means of amplifying the force or torque produced by the motor 22. Therefore, for a given load 26, a smaller motor 22 can be used. This, in turn results in less power consumption by the overall servomechanism. From a mathematical sense the amplification can be described by: EQU T.sub.out =T.sub.in *n (2)
These advantages make transmissions 24 very useful in many applications.
However, transmissions 24 also have disadvantages mainly due to the fact that the designer is adding another component to an already complex design. These disadvantages can be stated more distinctly as follows.
In most cases, the advantages of having a transmission 24 outweigh the disadvantages. However, in the applications where a system is very sensitive to velocity fluctuations, the mechanical errors in the transmission 24 can cause major problems in meeting design requirements. In fact, a major part of the design and development effort can be spent in an attempt to minimize the transmission-induced errors.
Therefore, it would be advantageous to build a servomechanism that did not require a transmission 24. Such a system is referred to as a direct drive servo. The traditional approach in designing a direct drive servo system employs the same components found in FIG. 1, minus the transmission 24. The problem that arises in a slow-speed application using a direct drive transmission 24 is that the feedback sensor 28 in conjunction with the digital controller 10 must meet a Nyquist data-sampling requirement. That is, the servomechanism must sample enough different angles in a given period of time (the sample rate of the servomechanism usually referred to as the Control Loop Sample Time) from the feedback sensor 28 to provide smooth operation of the servomechanism. This is one of the main reasons for using the quadrature signals out of an incremental encoder 28 since it offers an immediate 4.times. increase. In fact, numerous electronic schemes have been employed to further increase this resolution by electrical means. Unfortunately, these means are all only as good as the precision and accuracy of the original rising and falling edge relationships. The other disadvantages of these types of resolution increase methods are that they are complex and therefore add cost.
Therefore, motion control designers have attempted to use analog signal feedback sensors 28 that have an effective resolution of infinity. One way to accomplish this is to employ an optical encoder 28 that produces sine and cosine output signals. In the use of an incremental optical encoder 28, the designer has a device that has a base resolution. However, by interpolating the sine and cosine signals, the designer can approach a resolution of infinity.
Published articles on the Internet by Hathaway Motion Control, Hewlett-Packard (West (1994)), and Renco Encoders (Setbacken (1998)) discuss several methods of using sine and cosine waves to provide position and slow-speed motion control. In general, the methods employ the digital controller 10 to interpolate the sine and cosine to effectively increase the overall resolution of the incremental optical encoder 28, and employ some form of cycle counting to prevent runaway. For example if an incremental optical encoder 28 has 1000 cycles per revolution and the motion controller divides each cycle up into 500 divisions, the effective resolution is 500,000. Again, this fidelity is what gives the design the capability of running at slow-speeds since it can meet a sampled data Nyquist requirement.
The methods used to perform this interpolation or angle decoding are numerous. For example, Setbacken describes the use of the trigonometric relationship: sin(a+b)=cos(a)sin(p)+sin(a)cos(p). In this case, the base sinusoidal signals sin(a) and cos(a) are multiplied by phase shifted copies. See the article published on the Internet by Robert M. Setbacken. In the West article, a position control approach is used in which the actual crossover of sine and cosine waves (exactly the same as keeping track of the rising and falling edges of a quadrature encoder) is performed as a coarse measurement. The digital controller 10 then performs a fine interpolation using look-up-table or algorithm based on an arctangent function. The result is the ability to position at higher resolutions than the base resolution of the encoder 28.
These techniques work fine for high-resolution position control but still have a problem with smooth slow-speed control. Basically this is due to their method of controlling motor runaway. A motion control system is subject to noise, both mechanical and electrical. One of the main reasons for using a feedback sensor 28 is to take advantage of the fact that once a servomechanism is operating, it tries to fight these noise sources by filtering them out so that the control of the motion is maintained. If however, these disturbances are large, the digital controller 10 can lose track of which sine and cosine cycle is being measured (in the case of an incremental optical encoder, the encoder does not output an absolute position, the digital controller 10 determines the absolute position). If the controller 10 loses track of the cycle, it will produce an error signal that after being processed will cause the motor 22 to move the load 26 with a high-speed in one direction or the other. The result is loss of all control and a motor runaway condition. To control this phenomenon, the prior art systems have employed some form of cycle counting that is similar to the quadrature method discussed above as a coarse control on motor position. Determining the coarse and fine angles separately causes phase errors that lead to noise when the servo is used to control velocity. Because of the resulting noise, slow-speed control is still subject to unacceptable velocity fluctuations.
There is a need therefore for a servomechanism, using an incremental optical encoder having a sine and cosine output that can control the phenomenon of runaway, and which also has improved a slow-speed operation.